http://www.houstonasa.org/index1.html
This is the link to the Houston ASA website. Scott Geron would probably be your best bet for contacting regarding slowpitch in Houston. Most of these umpires listed on the website are on the Southeast side of Houston. Scott should be able to hook you up with whoever you need depending on which side of the city you end up staying at.
Feel free to contact me further if you have any questions. Ive lived here most of my life. Welcome to H-town!
Andy